A motor-vehicle latent-heat storage unit is typically provided between the vehicle engine and the heating system and has a pack of cells or heat pipes each comprised of an elongated plate-shaped tube filled with a latent-heat storage medium. In a chemical-phase system the storage medium is barium hydroxide and water (octahydrate) and the envelope surround it is made of oxygen-free copper or nickel-plated steel. For best heat exchange the metallic envelope is only several tenths of a millimeter thick.
The main problem with these systems is that the cells are subject to considerable stresses. They are heated and cooled during normal operation so that they will inherently expand and contract, and the internal phase changes are responsible for further changes in volume. Similarly the liquid inside them will expand and contract so that the internal pressure can vary quite a bit. Finally when employed in a motor vehicle they are subject to the dynamic shocks and stresses anything in a road vehicle has to withstand. Clearly it is difficult to build a cell that is sufficiently thin walled to ensure good heat transfer between its contents and the heat-transfer fluid surrounding it yet strong enough to withstand the stresses it is subjected to during normal use.
Accordingly it is known from German patent document 4,036,392 to use seamless tubing to have an envelope of maximum strength. Unfortunately making seamless tubing of the necessary thin wall thickness is extremely difficult. In this system the procedure is complex since the seamless tube is made from a flat plate of sheet metal into which an annular frame of a spacer tube is soldered. When the solder location is heated there are normally problems as the thin walls cannot withstand substantial pressure, as must be exerted by the iron during the soldering operation.